Structure-Reactivity Relationships in Ring-Opening Polymerization

9

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 9, 2016 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch009

Structure-Reactivity Relationships in Ring-Opening Polymerization STANISLAW PENCZEK, PRZEMYSLAW KUBISA, STANISLAW SLOMKOWSKI, and KRZYSZTOF MATYJASZEWSKI Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-362 Lódź, Poland

C o r r e l a t i o n s o f s t r u c t u r e s and r e a c t i v i t i e s for a n i o n i c and c a t i o n i c ring-opening p o l y m e r i z a t i o n are reviewed. The f o l l o w i n g t o p i c s are discussed: chemical s t r u c t u r e of a c t i v e species and t h e i r isomerism, determination of a c t i v e centers concentration, covalent vs i o n i c growth and c o r r e l a t i o n s between s t r u c tures of a c t i v e centers o r monomers and their reactivities. C o r r e l a t i o n s of s t r u c t u r e s and r e a c t i v i t i e s require f o r the ring-opening polymerization as well as f o r other i o n i c polymerizations approaches d i f f e r i n g from these i n r a d i c a l p o l y m e r i z a t i o n of the unsaturated monomers. This i s because i n r a d i c a l polymerization f r e e r a d i c a l s are the unique chemical s t r u c t u r e of the growing species and double bonds are the only chemical groups i n v o l v e d i n po l y m e r i z a t i o n (Ί), (_2) . Ring-opening polymerizations involve a v a r i e t y of the i o n i c growing species. Moreover, some of the hetero c y c l i c monomers may react ambidently and, t h e r e f o r e , pro duce chemically isomeric s t r u c t u r e s of a c t i v e centers. P o l y m e r i z a t i o n of lactones or p o l y m e r i z a t i o n of s u b s t i t u ted α-oxides, both with two p o s s i b l e ways of ring-opening, are the t y p i c a l examples. Thus, the a c t u a l chemical s t r u c t u r e s have to be de termined f i r s t and then t h e i r proportions and c o n t r i b u t i o n s i n the chain growth have to be e s t a b l i s h e d . The f u r t h e r step i s the determination of the rate constants of the elementary r e a c t i o n s i n v o l v i n g a l l of these species that have to be c o r r e l a t e d . C o r r e l a t i o n s are made f i r s t f o r a given monomer pro pagating with various growing s p e c i e s , then r e a c t i v i t i e s of monomers belonging to the same c l a s s of chemical compounds are determined and e v e n t u a l l y c o r r e l a t i o n between monomers with d i f f e r e n t heteroatoms can be given. 0097-6156/85/0286-0117$06.00/0 © 1985 American Chemical Society

In Ring-Opening Polymerization; McGrath, James E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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118

This above discussed c o r r e l a t i o n s r e q u i r e that iden t i c a l chemical mechanisms are compared. For i n s t a n c e , the c a t i o n i c polymerizations of h e t e r o c y c l i c s are known t o proceed by SN1 , S^2, and Aq2 mechanisms. Besides, there are two d i f f e r e n t SN2 mechanisms, and both can involve the same monomer, namely proceeding with onium ions or with a c t i v e d monomer (3). T h i s paper d e s c r i b e s problems o u t l i n e d above, methods of determination of the chemical s t r u c t u r e s i n both anio n i c and c a t i o n i c ring-opening p o l y m e r i z a t i o n s , e q u i l i b r i a between d i f f e r e n t a c t i v e s p e c i e s , the corresponding me chanisms of propagation and r e l a t e d rate constants o f propagation on these s p e c i e s , and f i n a l l y the a v a i l a b l e correlations. Determination o f the chemical s t r u c t u r e s of the growing species A n i o n i c p o l y m e r i z a t i o n . For some h e t e r o c y c l i c monomers the unique chemical s t r u c t u r e o f the growing species follows u n e q u i v o c a l l y from the monomer s t r u c t u r e . However, i n many cases isomeric s t r u c t u r e s have to be taken into account. For i n s t a n c e , f o r symmetrical monomers, l i k e t h i e t a n e , the carbanion but not the t h i o l a t e anion was proposed Ç4) . Unsymmetrically s u b s t i t u t e d monomers can p r o v i d e a c t i v e species by a- or 3- r i n g s c i s s i o n . Unusual s t r u c t u r e o f a c t i v a t e d monomer was proposed f o r NCA and lactams. These s t r u c t u r e s can not be d i s t i n g u i s h e d by spectrophotometry methods, and a p p l i c a t i o n of ^H- o r 1^C-NMR looks more promising. We have r e c e n t l y elaborated a method based on the anion capping with C1P(0)(OC6H5)2, followed by determina t i o n o f the s t r u c t u r e o f the parent anion i n 31p{1n}-NMR, and comparing chemical s h i f t s with these of the indepentl y s t u d i e d model compounds (5). Some examples are given i n Table 1 ; below i s the general scheme: 0 0

RX ,

M t ® + C1P (0) ( O C H ) 6

5

•

2

RX-P(OC H ) 6

5

2

+ MtCl

(1)

where X=heteroatom and Mt=e.g. Na , Κ Thus, t r a p p i n g provides a p o s s i b l e way o f determi ning the isomeric s t r u c t u r e s during p o l y m e r i z a t i o n , measuring t h e i r p r o p o r t i o n s and t h e i r rates o f i n t e r c o n version. The corresponding s t r u c t u r e s of the growing species have been e s t a b l i s h e d by comparing the observed chemical s h i f t s of the trapped products with these o f the model compounds. Thus, trapped CH30© CH3CH(C6H5)0©, C H C H C H 2 U , and C H - C H ( C H ) S O give the f o l l o w i n g chemical s h i f t s : -11.5, -13.1, -12.7 ( i n THF) and +19.0 ppm 6 ( i n CfcH^). The carboxylate and s i l a n o l a t e models e>

6

5

2

3

3


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Structure- Reactivity Relationships


both lead to the same chemical s h i f t equal to -26.3 ppm 6 (THF) and c h a r a c t e r i s t i c f o r [KC6H5O) 2 (0)P] 2O (5). These and r e l a t e d methods allowed us r e c e n t l y to r e evaluate the s t r u c t u r e o f a c t i v e centers i n a n i o n i c p o l y m e r i z a t i o n of simple, u n s u b s t i t u t e d l a c t o n e s , β-propiolactone. The r a t i o n a l e was put forward i n terms of stereo e l e c t r o n i c f a c t o r s to e x p l a i n why 3-propiolactone propa gates on carboxylate and ε-caprolactone on a l c o h o l a t e anions. T h i s i s shown i n scheme below :

The broken arrow i n the p i c t u r e s above i n d i c a t e s the hampered d i r e c t i o n o f attack f o r the approaching nucleophile. Table I.

Monomers, s t r u c t u r e s o f the growing species and trapped anions, according to ^ P O H I - N M R (5) .

Monomer H

f 2

C H

product of trapping .-CH OP(0) ( O C H )

2?

2

P2ÇHO

6

5

δ

by Ρ ppm

-11.5

2

.-CH2CHOP(0)(OC6H5) 2 -13.0 .-^HCH 0P(0) ( O C H ) -12.6 2

6

5

2

Growing species -CH 0^ o

-CH2 ?H0© -CHCH 0Θ è 9

2

Q CH,CH,OC

I

2

0 ..-CH fi0P(0)(OC H ) 2

1

2

5

/β 2

-CH Ci© ^0 7

L

pyrophosphate

(CH )- 0 2

6

-26.0

.{CH ) 0P(0)(0C H )

5

2

5

6

5

CH .. .-SiOP (0) (OC.HJ CH

2

-11.5

.-CH OP 2

3

|HCH ) SiO|| 3

2

2

0

5

.-SiO@ tH

9

L

3

3

\

pyrophosphate CH CH,ÔHS

-26.3

3

.-CH CHSP(0) ( 0 C H ) CH 2

6

3

5

2

+18.9

.-CH^HS" CH 3

Q u a n t i t a t i v e determination of the c o n c e n t r a t i o n of macroanions i n the a n i o n i c p o l y m e r i z a t i o n of h e t e r o c y c l i c s i s based on the same approach of end-capping with P-containing compounds.


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POLYMERIZATION

Cationic polymerization. S i m i l a r l y , trapping i n the cat i o n i c p o l y m e r i z a t i o n with R3P, where R=alkyl or a r y l , allows one to determine the s t r u c t u r e of the parent ca t i o n s , shown below f o r onium ions (6), (7): 0/CHp\ . ..-CH X J + 2

CH


PR

x

θ • . ..-CH -X-CH 'Vb'vCH PR

3

2

2

2

(3)

3

2 - ^ C a n i o n omitted)

These methods are r e l a t e d to the b e t t e r known trapping of r a d i c a l s , trapping of carbanions elaborated by Szwarc (8) and, more r e c e n t l y , end-capping with phenolates i n c a t i o n i c p o l y m e r i z a t i o n (9,10,11). The phosphine i o n - t r a p p i n g , i n c o n t r a s t to the methods using UV spectrophotometry f o r f u r t h e r i d e n t i f i c a t i o n , provides information about the f i n e s t r u c t u r e of the growing s p e c i e s . More information can f o l l o w from mul t i p l i c i t i e s of the P-NMR spectra. Some examples of the a p p l i c a t i o n of the trapping of c a t i o n s with phosphines are given below : 31

Table

II.

Monomer

Monomers, and s t r u c t u r e s of the r e l a t e d growing species and quaternary phosphonium s a l t s (trapped cations) (6). Product

— ppm

Growing species

P

of trapping

© ...-0{CH > -P(C H )

3

23.8

©^~x ...-CH -0 - J

© ...-0{CH > -P(C H )

3

23.4

©^-^ ...-CH -0 5 J

2

2

3

6

4

5

6

5

2

2

® ...-0{CH } -P(C H ) 2

6

6

5

-CH OCCH CH P(C H ) 2

2

2

6

5

23.0

3

3

23.4

...-CH 0*^C2

0 ©

©

. ..-CH OCH -P(C H ) 2

2

6

5

3

16.7

M

...-CH -OCH " 2

2

It has p r e v i o u s l y been shown i n our laboratory that i n the t e r p o l y m e r i z a t i o n of oxetane, THF and oxepane the a c t i v e end group of a l l three growing species could be simultaneously observed (6) .


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121

P a r t i c u l a r l y important i s the e s t a b l i s h e d s t r u c t u r e of the growing species i n the p o l y m e r i z a t i o n of 3-propiol a c t o n e , which d i f f e r s from the accepted e a r l i e r acylium c a t i o n . The t e r t i a r y oxonium i o n s t r u c t u r e , observed by us a l s o f o r ε-caprolactone, has been confirmed by other methods (1 2) . 1H an3 C-NMR have also been s u c c e s s f u l l y used i n determination of s t r u c t u r e s of onium ions as the growing species. This has already been reviewed by us ( 1 3 ) . 13


Isomerism o f the i o n i c a c t i v e centers The chemical s t r u c t u r e s described above o f a c t i v e i o n i c centers have been considered as the unique ones. Thus, although they may e x i s t i n s e v e r a l p h y s i c a l forms as various i o n - p a i r s , t h e i r aggregates or " f r e e " i o n s , but there i s only one chemical s t r u c t u r e they propagate on. We r e c e n t l y observed however systems i n which i o n i c growing species d i f f e r i n g i n chemical s t r u c t u r e c o e x i s t i n these systems and p a r t i c i p a t e i n the chain growth. In a n i o n i c p o l y m e r i z a t i o n , 3 - p r o p i o l a c t o n e i n i t i a t e d with potassium a l c o h o l a t e , g i v e s , i n i n i t i a t i o n , b o t h a l c o holate and carboxylate anions. A l c o h o l a t e ions i n every next step convert p a r t i a l l y into carboxylate whereas car boxylate reproduce themselves q u a n t i t a t i v e l y . Thus, a f t e r a few steps only carboxylate anions are l e f t ( 1 4 ) . Related s i t u a t i o n was observed i n the p o l y m e r i z a t i o n of styrene oxide ( 1 5 ) . Here, however, i t i s only due to the s t r u c t u r e o f the i n i t i a t o r used. Thus, when i n the i n i t i a t i o n step both secondary and primary a l c o h o l a t e anions are formed, due to the low s t e r i c requirements, i n the next step apparently only the attack on the l e a s t s u b s t i tuted carbon atom takes place and already i n the second step e x c l u s i v e l y secondary a l c o h o l a t e anions are present. In these two systems e v e n t u a l l y macromolecules are formed by one kind of a c t i v e species winning e a r l y enough i n competition with the other s p e c i e s . However, i t has been observed i n t h i s l a b o r a t o r y , p a r t i c u l a r l y i n the c a t i o n i c p o l y m e r i z a t i o n , e s p e c i a l l y i n the p o l y m e r i z a t i o n of c y c l i c a c e t a l s ( 1 6 ) and o r t h o e s t e r s (J_7) , that two or more chemically d i f f e r e n t kinds of a c t i v e species may c o e x i s t throughout the whole p o l y m e r i z a t i o n process, t h e i r proportions may depend ( c y c l i c a c e t a l s ) on the mo nomer conversion. Thus, i n the p o l y m e r i z a t i o n of c y c l i c a c e t a l s the carbenium-oxonium e q u i l i b r i a have to be taken i n t o account (JU>) : θ ...-CH OCH 2

where 0

V

^ 2

+ 0^

Θ.... .. . - C H O C H O 2

2

N

(4)

i s a monomer molecule or another macromolecule.



122

More r e c e n t l y one of us with Szymanski observed that the a c t i v e species h o l d i n g monomer molecule can isomerize and the f o l l o w i n g e q u i l i b r i u m was d i r e c t l y observed by 'Hand 13c-NMR i n model compounds (18):

CH -ÇH 2

-

2

CH OCH -yD 5 Jd 2 3

2

C H

L

N

2

0

CH

^

^

N

° CH2° CH -CH


9

CH —CH 9

CH /sJ

0

CH 2

CH OCH — ν, • ^ 3

2

0

^

^-—

ι 2

CH

9

9

z

ffil

v.ix —w ^ 3

ι C

(5)

9

0

9

χ

CH

2

CH

9

CH

9

L

H

2 2

As i n d i c a t e d by the d i r e c t i o n of the arrows the isomeric 7-membered oxonium ion dominates i n the p o l y m e r i z a t i o n of the 5-membered 1,3-dioxolane whereas i n the polymeriza t i o n of the 7-membered 1,3-dioxepane c a t i o n a t e d monomer dominates. This i s apparently due to the d i f f e r e n c e s i n s t r a i n o f the i n v o l v e d r i n g s . K i n e t i c a n a l y s i s o f the po l y m e r i z a t i o n of these two monomers has shown that the isomeric (enlarged) oxonium ions can be t r e a t e d as the k i n e t i c a l l y dormant s p e c i e s ; propagation and depropagat i o n on these species proceed with almost i d e n t i c a l r a t e s . This explains why f o r the same s t a r t i n g concentration of i n i t i a t o r , as observed by P l e s c h (19), 1,3-dioxepane poly merizes over 100 times f a s t e r than~T,3-dioxolane. This i s because the p r o p o r t i o n of the p r o d u c t i v e l y a c t i v e species i s higher f o r the former than f o r the l a t t e r monomer. Covalent

growing species

C l o s e l y r e l a t e d to the i o n i c p o l y m e r i z a t i o n of heterocy c l i c monomers i s , what we can c a l l , pseudoionic polymeri z a t i o n (or sometimes, perhaps, c r y p t o i o n i c ) . We use the p r e f f i x pseudo- i n the same meaning as i t was f i r s t used i n the v i n y l c a t i o n i c p o l y m e r i z a t i o n . I t means that propagation a c t u a l l y proceeds on the covalent species that could have been i n e q u i l i b r i u m with t h e i r i o n i c counterparts. Several systems f a l l i n g to t h i s category have r e c e n t l y been described f o r both a n i o n i c and c a t i o n i c p o l y m e r i z a t i o n of h e t e r o c y c l i c s . In the a n i o n i c pro cesses d e r i v a t i v e s of Zn or A l a l k y l s or a l c o h o l a t e s are b e l i e v e d to f u n c t i o n t h i s way. However, f o r none of these systems the absence of i o n i c c o n t r i b u t i o n was shown. Two c a t a l y t i c systems are of p a r t i c u l a r i n t e r e s t , namely the -Zn-0-ΑΚ systems (_20) and>Al-alkyl modified by bulky porphyrin d e r i v a t i v e s (21). Both are discussed i n t h i s volune and both have been c l e a r l y shown to produce l i v i n g systems. The former with ε-caprolactone and the l a t t e r


PENCZEK ET AL.

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123


with ethylene oxide, propylene oxide and 3 - p r o p i o l a c t o ne (22) . As i n d i c a t e d above, i n the a n i o n i c processes only " e i t h e r - or" s i t u a t i o n was observed, i . e . when covalent species are present no ions i n e q u i l i b r i u m were detected. Covalent a c t i v e species i n the c a t i o n i c p o l y m e r i z a t i o n . In the c a t i o n i c p o l y m e r i z a t i o n s e v e r a l systems were studied, i n which both covalent and i o n i c growth have been simultaneously s t u d i e d . For the f i r s t time the cova l e n t growth was described f o r oxazolines by Saegusa ( 2 3 ) . In the p o l y m e r i z a t i o n of THF the presence of cova l e n t species was assumed by Smith and Hubin (24) and s h o r t l y a f t e r the covalent species were d i r e c t l y observed i n our l a b o r a t o r y ( H-NMR) (25) as w e l l as by Saegusa 1

1

( 9F-NMR)

Ç26)

and

Pruckmayr~T13c-NMR)

(27).

ÏH-NMR

c l e a r l y showed the existance of two d i s t i n c t s p e c i e s , covalent and i o n i c , with t h e i r c h a r a c t e r i s t i c chemical s h i f t s i d e n t i c a l to model compounds. In the p o l y m e r i z a t i o n of h e t e r o c y c l i c monomers, the covalent species i n e q u i l i b r i u m with t h e i r i o n i c counter p a r t s were observed d i r e c t l y , thus the corresponding e q u i l i b r i u m constant could be determined f o r polymeri zing systems. There are two r e a c t i o n pathways p o s s i b l e f o r the i o n i z a t i o n r e a c t i o n : .. .-CH 0(CH ) A 2

2

n

(6)

The e x t e r n a l i o n i z a t i o n i n v o l v e s a d d i t i o n of the monomer molecule to the covalent a c t i v e species and, thus, means the covalent propagation. The c o n t r i b u t i o n of each o f the two mechanisms shown i n scheme (6) and operating simultaneously may be estimated on the b a s i s of the dependence of the | i o n | / I e s t e r I r a t i o on conversion. For unimolecular i n t e r n a l r e a c t i o n t h i s p r o p o r t i o n should be independent of mono mer c o n c e n t r a t i o n (thus conversion) while f o r the bimolec u l a r , e x t e r n a l i o n i z a t i o n the p r o p o r t i o n of ions should decrease with conversion. I t was shown that f o r the most thoroughly studied system, i . e . p o l y m e r i z a t i o n of THF, the i n t e r n a l i o n i z a t i o n dominates (28). More recent r e s u l t s i n d i c a t e that i n the polymeriza t i o n of the 7-membered c y c l i c ether: oxepane (Ox)>both i n t r a - and i n t e r m o l e c u l a r i o n i z a t i o n s have to be


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POLYMERIZATION

4

1

considered. Thus i n CH3NO2 solvent at 25° 1 ^ = 2 . 3 · 1CT s" and k i=1.35·10-4 mol~i»l»s~1 meaning that both processes proceed with the same rates f o r |0x|=1.7 mol»l"^ ( e f f e c t i v e monomer c o n c e n t r a t i o n ) . For the discussed e a r l i e r THF case the e f f e c t i v e monomer c o n c e n t r a t i o n would be above 100 mol«l~* i . e . much above the c o n c e n t r a t i o n which may be achieved even i n bulk (29). e


R e a c t i v i t i e s of covalent a c t i v e species In the p o l y m e r i z a t i o n o f h e t e r o c y c l i c compounds rate constants of propagation on covalent species were deter mined f o r s e v e r a l systems and compared with the c o r r e s ponding r a t e constants of i o n i c growth. In the polymeri z a t i o n of THF, k p = 5 « 1 0 - mol-1-l«s-1 i n CH3NO2 at 25°; the s i m i l a r value 3·10" mol~1»l-s-1 was measured i n the Ox p o l y m e r i z a t i o n . Although the values of the rate cons t a n t s of covalent propagation are c l o s e to each other, the c o n t r i b u t i o n of covalent growth i s considerably d i f f e r e n t because the corresponding i o n i c rate constants are d i f f e r e n t : kpi=2.4«10- m o l - ^ l - s " f o r THF and 1.3 · 1 0 ~ mol-1·1·s~1 for Ox. The observed r e l a t i o n s are due to the low s t e r i c requirements of covalent growth and the much l a r g e r r o l e of s t e r i c hindrance f o r i o n i c growth, as discussed by us i n Ref. 13: 4

C

4

2

1

4

Macroion-pairs

and macroions

Below, i n Table III some t y p i c a l data on d i s s o c i a t i o n of the macro- i o n - p a i r s f o r both a n i o n i c and c a t i o n i c r i n g -opening p o l y m e r i z a t i o n are given. There i s a number o f s i m i l a r i t i e s i n behaviour of macroions d e r i v e d from various monomers. Thus, macroion- p a i r s of l i v i n g poly(ethylene oxide) and p o l y c a p r o l a c t o ne i n THF solvent with K ® c a t i o n s , both have very low Κβ. D i s s o c i a t i o n of l i v i n g poly-B-propiolactone, with carbox y l a t e growing anion and crowned K® counterion, i n which e l e c t r o s t a t i c i n t e r e a c t i o n w i t h i n the i o n - p a i r i s much weaker than i n a l c o h o l a t e i o n - p a i r s , resembles t h i s of the t e r t i a r y oxonium i o n s . For both systems i n CH2CI2 solvent Kj) i s approximately equal at 10~5 mol»l"1, i . e . 10 times l a r g e r than f o r a l c o h o l a t e i o n - p a i r s i n THF solvent. ;

5


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D i s s o c i a t i o n constants o f some macroion-pairs i n the a n i o n i c and c a t i o n i c ring-opening poly merization

Monomer

Growing species

CH CH 0

•·«-CH2CH2O

(CH ).OCO

...-CH CH 0®

2j

9

9

9

l

2

9

L

..

o

L

D

2

.-CH CH O 2

0

2

...-CH CH O

2

K®

THF

1.8«10~ 30

K®

THF

4.Ί0(200)

Cs®

THF

2.7·10~

2

0

2

K®/|222|

THF

2.0·10'

K®

THF

2.MO"

CH CH(CH )S

. . .-CH CH(CH )S^

CH,CH OCO 1 1

•·»-CH2CH2C^B

K®/DB18C6

^CH ) 0

- Ό

"A

2

3

9

2

3

1C

C H

2

C 1

2

C H

2

C 1

2

2

2

4

10

11 ΊΟ

30

3

SbFf

(CH ) 0 6

5·10'

7

30

8

32 33

5

5

CH N0

2

1

mol-r

\

pH CH 0

2

Solvenlt K , 2 5 °

i

CH CH 0

1

2

0

Counter-ion

Réf. Ι

Table I I I .

1 2



3·10" (00) _ 2·10-

34

3

2

5

3.1·10~ (0°) _ 35 C H N0 1.6.10" C H

2

C 1

2

3

6

h

...-CH -^ 2

I®

5

2

C H N0 3·10" 6

5

2

2

36

The d i s s o c i a t i o n constants d i s c u s s e d above were determi ned from the conductometric data according to Fuoss. The large m a j o r i t y o f the c a t i o n i c processes are w e l l d e s c r i bed by a simple scheme o f i o n - p a i r d i s s o c i a t i o n ; the Κβ determined f o r both the low molecular models and the h i g h polymer f i t t e d with the i o n - p a i r at the end g i v e s i m i l a r r e s u l t s . The high n u c l e o p h i l i c i t y of monomer, s t r o n g l y s o l v a t i n g the c a t i o n , and l a r g e s i z e o f anions decrease the i n t e r a c t i o n w i t h i n the i o n - p a i r i n both thermodynamic and k i n e t i c sense. In the a n i o n i c p o l y m e r i z a t i o n the s i t u a t i o n i s d i f f e r e n t . The negative charges are h i g h l y concentrated at the chain end at l e a s t f o r a l c o h o l a t e and t h i o l a t e anions, a l k a l i metal c a t i o n s u s u a l l y used as counterions have


RING-OPENING

126

POLYMERIZATION


smaller s i z e , and s t r o n g l y i n t e r a c t with anions. Therefo r e , the e l e c t r o s t a t i c a t t r a c t i o n w i t h i n an i o n p a i r i s stronger and KD extremly low. Besides, these i o n - p a i r s are l e e s s u s c e p t i b l e to s o l v a t i o n and s t r o n g l y s e l f - a s s o c i a t e i n t o aggregates. Thus, a n a l y s i s of the f i n e s t r u c ture of ion p a i r s on the bases of Fuoss equation as w e l l as i t s a p p l i c a b i l i t y i n the a n a l y s i s of d i s s o c i a t i o n i s less straightforward than f o r the s o l v a t e d (or solvent separated) i o n - p a i r s . The l a t t e r do not change the degree of s o l v a t i o n i n d i s s o c i a t i o n : 0

...-X®(Mt-nS)®—...-X *

(Mt-nS)

0

(6)

whereas the former may require at l e a s t two d i s c r e t e steps f o r d i s s o c i a t i o n , namely the p r e l i m i n a r y s o l v a t i o n and then d i s s o c i a t i o n of the thus solvated i o n - p a i r . Fuoss (37) has r e c e n t l y s t r e s s e d that the determination of the oTstance between ions f o r such a multistep process may r e q u i r e an approach d i f f e r i n g from the a p p l i c a t i o n of a simple dependence of KD on the d i e l e c t r i c constants or r e c i p r o c a l of the absolute temperature ( i . e . the Fuoss equation). Aggregation of i o n - p a i r s has been demonstrated i n the p o l y m e r i z a t i o n of ethylne oxide (...-CH20OK® i n THF s o l v e n t ) ; apparently c y c l i c trimers of ion p a i r s domina t e , formed with the e q u i l i b r i u m constants equal approx. to Ι Ο ^ Ή Ο l - m o l ~ . This value was determined from the a n a l y s i s of the k i n e t i c s of p o l y m e r i z a t i o n (30). Polyme r i z a t i o n of ε-caprolactone with Na® as counterion i n THF solvent also shows the 1/3 dependence of the rate of poly m e r i z a t i o n on the t o t a l concentration of a c t i v e species (38) whereas with K® counterion p a i r s do not aggregate i n THF (J2)· However i n the p o l y m e r i z a t i o n of l e s s p o l a r dimethyl siloxane trimer (D3) (Na® cation) i o n - p a i r s e f f i c i e n t l y aggregate i n THF (39). The observed concen t r a t i o n dependences s t r o n g l y i n H i c a t e the formation aggregates but there are no other more d i r e c t proofs of t h e i r existance. According to Kazanski (40), a l l of the attempts to determine the s t a t e of a s s o c i a t i o n from the v i s c o s i t y measurements have to be considered as unsucces s f u l a f t e r c l o s e r examination of the c o n d i t i o n s of measu rements and r e l a t e d t h e o r e t i c a l f u n d a m e n t a l s - p a r t i c u l a r l y when the 3/4 law derived f o r concentrated s o l u t i o n s (or polymer melts) i s being a p p l i e d to the d i l u t e s o l u t i o n s i n which polymerization proceeds. Increasing the solvent p o l a r i t y i n both a n i o n i c and c a t i o n i c systems increases s i g n i f i c a n t l y Krj. Thus, Kj) of a l c o h o l a t e macroion-pairs from poly(ethylene oxide) with K® i n DMSO solvent i s equal to 4.7·10" m o l ' l " and Kn i n the p o l y m e r i z a t i o n of THF i n CH3NO2 s o l v e n t equals 10* mol»1" . The same e f f e c t i s observed when i n the anionic poly m e r i z a t i o n l a r g e r cations are introduced. P a r t i c u l a r l y when crowned or cryptated cations are used. 7

2

2

2

1

Ί


9. PENCZEK ET AL.

Structure-Reactivity Relationships

i n propagation

There are a few systems f o r which, using to e s t a b l i s h proportions of macroion-pairs and macroions, the rate constants of propagation on these species were determined. In the c a t i o n i c p o l y m e r i z a t i o n o f THF, OXP, and more r e c e n t l y conidine, i t has been shown that kp=k^ (_36) · This was explained by assuming weak i n t e r a c t i o n s o f counter ions w i t h i n the i o n - p a i r s , due to d i s s i p i t a t i o n of the p o s i t i v e charge i n the onium i o n s , as well as by the stereochemical course of the propagation step (bordeline Sjyj2) i n which the monomer approach hardly requires the p u l l i n g apart o f the anion. Table IV.

Rate constants of propagation i n a n i o n i c poly m e r i z a t i o n of h e t e r o c y c l i c compounds +

Monomer polymerization conditions

k Ρ

A c t i v e species

mol

3

2

32

2.5·1θ"

3

3.8

32

1

-

32

5.6

32

2.5-ΊΟ"

.-C^CHCCH^^Na® .-CH CH(CH0S^Cs®

2

)

0

. . .-CH^CH(CH^)^ Na |222|

3

32

1.67

Ί.22.10"

6>

9

Ui(CH *- oJ

-

2

1

. . . - C H C H 0 K ® 222 2

32

2

...-CH2CH20®Cs

THF, -30°C

*l*s

4.8-ΊΟ"

... —CH2CH2Û^K THF, 20°C

k" Ρ

S i (CH ) 2 0 ^ 1 * |211| 3

Réf.


Macroions and macroion-pairs

127

2.3-10" 11.9

32

1.4

Benzene, 20°C (CH f OCO 2

2

CH C1 , 2

2

0

. . .-CH2CH COO K®DB18C6 2

4

7.0·10~

Ί

1.6·10" 33

25°C

(CH^OCO

0

. ..-C(0) ( C H ) O K ® 2

5

4.7

-

31_

THF, 20°C

In the a n i o n i c p o l y m e r i z a t i o n there are three monomers only that have been studied i n more d e t a i l , namely ethy lene oxide, propylene s u l f i d e , and 3-propiolactone. Some




128

p r e l i m i n a r y data on ε-caprolactone have become a v a i l a b l e more r e c e n t l y . Polymerizations of ethylene oxide and propylene s u l f i d e were reviewed s e v e r a l times by the authors of the o r i g i n a l r e s u l t s , namely the P a r i s and the Moscow groups (32), (40). One of us with Kazanski reviewed r e c e n t l y the recent clata, i n c l u d i n g a l s o p o l y m e r i z a t i o n of lactones (30) . In the p o l y m e r i z a t i o n of ε-caprolactone with K® counterion i n THF propagation proceeds e x c l u s i v e l y on the i o n - p a i r s (31). These i o n - p a i r s p r a c t i c a l l y do not d i s s o c i a t e and do not aggregate at the p o l y m e r i z a t i o n c o n d i t ions (temp, from 0 to 20°, THF, |eCL| =0.5 m o l » l " ) . The comparison of the r a t e constants of propagation on the a l c o h o l a t e i o n p a i r s with K® counterions i n the homopolym e r i z a t i o n of ε-caprolactone_(ki (20°)=4.7 m o l " l s (31) with that of oxirane (k£ (20°)=4.8·10" m o l - · 1 · s " (TZ) r e f l e c t s the much higher r e a c t i v i t y of the former monomer. Presumably t h i s i s because the higher r i n g s t r a i n of oxirane, i n comparison with that of ε-caprolactone, i s overweighed by the higher r e a c t i v i t y of the e s t e r group i n eCL i n comparison with the r e a c t i v i t y of the ether linkage. 1

o

l e

2

e

1

_ 1

1

S o l v a t i o n phenomena H e t e r o c y c l i c monomers and polymers present i n t h e i r p o l y m e r i z a t i o n s t r o n g l y i n t e r a c t with the growing s p e c i e s . T h i s i s manifested i n f a c t s already d e s c r i b e d i n t h i s paper. C a t i o n i c p o l y m e r i z a t i o n . In the c a t i o n i c p o l y m e r i z a t i o n of c y c l i c ethers, s u l f i d e s , or amines i n CH2CI2 or even i n n i t r o s o l v e n t , monomers and r e s u l t i n g polymers are the most n u c l e o p h i l i c components of the system. Therefore, e x p l a i n i n g equal r e a c t i v i t i e s of macroions and macroion- p a i r s i n the c a t i o n i c p o l y m e r i z a t i o n of h e t e r o c y c l i c monomers, we assumed that both i o n - p a i r s and ions are s o l vated by monomers themselves. This decreases the e l e c t r o s t a t i c i n t e r a c t i o n w i t h i n the i o n - p a i r s . However, more d e t a i l e d a n a l y s i s of ΔΗρ and ASÎ ( a c t i v a t i o n parameters of propagation) revealea that these monomers (at l e a s t THF and oxepane) do not polymerize merely i n c l u s t e r s of mo nomer and polymer (8), but that solvent molecules are also present i n the immediate v i c i n i t y of the a c t i v e species (34), (41). This c o n c l u s i o n was based on the f a c t that ΔΗρ and AS^~measured i n various solvent d i f f e r t r e mendously; e.g. AHJj f o r THF i n THF solvent equals 14.0 kcal»mol-' whereas i n THF/CCI4 mixture equals 8.6 k c a l - m o l " . Due to the compensation by the a d j u s t i n g chan ges of ASJj the corresponding rate constants measured i n these s o l v e n t d i d not change more than two- three times. 1

Anionic p o l y m e r i z a t i o n . In a n i o n i c p o l y m e r i z a t i o n ethylene oxide, propylene s u l f i d e or t h e i r corresponding


PENCZEK ET AL.

9.


129

polymers are able to s o l v a t e c a t i o n s . This s t a t e of s o l v a t i o n should d i f f e r with temperature and s i n c e s o l v a t i o n i s exothermic, the lower the temperature the higher the c o n t r i b u t i o n of s o l v a t i o n to the energetics of r e a c t i o n s . Thus, d i s c u s s i n g any c o r r e l a t i o n between s t r u c t u r e and r e a c t i v i t y not only the e l e c t r o n i c and s t r u c t u r a l e l e ments of the a c t i v e species and monomers but also the s o l v a t i o n phenomena should be taken i n t o an account. Below, i n Table V the s o l v a t i o n power of ethylene oxide, propylene oxide, THF, and p o l y ( e t h y l e n e o x i d e ) , M =6000, are compared. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 9, 2016 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch009

n

Table V.

E q u i l i b r i u m constants of complexation of Na ethers and p o l y e t h e r solvents at 25° (30). -1 Κ , 1· mol η* Ί

η

Ligand

by

ethylene oxide

1

0. 41

propylene

oxide

1

0. 36

THF

1

p o l y ( e t h y l e n e oxide) 6000

6

0. 69 3000

The e q u i l i b r i u m constants l i s t e d i n Table V, measured by using N a and Cs-NMR i n d i c a t e that i n the polymeriza t i o n of ethylene oxide the Oolymer formed should s t r o n g l y and s e l e c t i v e l y s o l v a t e Na®^counterion. T h i s i s a l s o true f o r K® and Cs®^ c a t i o n s ; the corresponding K f o r p o l y e t hylene oxide) are equal to 500 and 200 1 mol-1. S o l v a t i o n of c a t i o n s by p o l y ( e t h y l e n e oxide) chain i s h i g h l y cooperative, showing the phenomenon "nothing or e v e r y t h i n g , i . e . the c a t i o n i s e i t h e r not s o l v a t e d or f u l l y solvated using i t s complete c o o r d i n a t i o n a b i l i t y : 2 3

133

n

11

X

X ° ο

•

V.Mt*JÏ2-

M t ® 3 = n

^

/

ι-

0

Mt / 0^

^

0 0

—

Structure-Reactivity Relationships in Ring-Opening Polymerization

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